I used chipmixer in March. I thought the private key he gave me would be deleted, but it turned out that it was not deleted. On March 16, my btc was transferred by the police. I didn't commit a crime, it's just for privacy, so is there any way to recover my btc 
No, there is not. Although your session might have been too close to the date of seizure for your private keys to have been wiped, there's a chance he never actually deleted any private keys, at all.
It is 7TB. If backups were done that include the private keys, that was a big mistake. I can guarantee you that these 7TB include private keys, since I have seen funds withdrawn from ChipMixer (imported private keys)) disappear on the day of the ChipMixer takedown.
Which would mean this was not true:
After you have received private key and destroyed session, we do not have it anymore.
Although OP didn't provide hard evidence that supports his allegations, I don't see a reason why ChipMixer shouldn't be tagged & flagged at this point. Especially to prevent others from falling victim to phishing scams (not everybody knows CM is no longer operational).